This invention pertains to image converters which can be used to convert non-visible radiation into visible images, and which can also be used to convert relatively weak projected images in the visible portion of the electromagnetic spectrum into amplified images of greater intensity. Such image converters conventionally utilize a photocathode upon which, either directly or indirectly, incident radiation can impinge, and from which photocathode electrons can be emitted in proportion to the amount of impinging radiation. Such image converters also conventionally utilize a luminescent screen which is maintained at a higher potential than the potential of the photocathode, in order to cause the electrons emitted from the photocathode to strike the luminescence screen and to thereby produce a visible image.
Conventionally, the photocathode is mounted on an entrance window, and the luminescent screen is mounted on an exit window. The exit window is made essentially transparent to the luminescence of the luminescent screen, in order to allow the image produced on the screen to be detected and subsequently utilized. In such image converters, the volume between these two windows may be evacuated.
In such image converters, it has been found that unintentional electron emission can cause avalanche discharge and can therefore cause arcing when the interior surface of the housing which supports the windows is perpendicular to the direction of equipotential lines of the electric field existing between the photocathode and the luminescent screen. Such arcing is undesirable.
Moreover, the bulkiness of some known image converters and the requirement that such image converters be supplied by more than one relatively high D.C. voltage supply makes such image converters difficult to use. It would therefore be advantageous to provide an image converter which required only one relatively low D.C. supply voltage for operation, and which would be so designed as to prevent arcing caused by avalanche discharge.